The present invention relates to improvements in the art of preparing moldable reinforced polyesters characterized by being tack free and by little or no tendency towards shrinkage during the curing thereof, and to polyester compositions which when cured produce molded articles having superior surface smoothness; that is, "low profile", or little variation in surface smoothness. More particularly it relates to polymerizable polyester compositions comprising a compatible mixture of an .alpha., .beta.-unsaturated polyester, an .alpha., .beta.-ethylenically unsaturated monomer copolymerizable therewith, and a saturated polyester containing three or more terminal carboxylic acid groups, and having a hydroxyl number of less than about 10.
It has long been known to prepare linear polyesters by the reaction of dihydric alcohols and ethylenically unsaturated dicarboxylic acids, and that such unsaturated polyesters are capable of addition polymerization with unsaturated monomers to form thermoset polymers. These types of copolymers are widely disclosed in this art, for example, in U.S. Pat. No. 2,255,313 to Ellis, and others.
It is known also to admix such linear ethylenically unsaturated polyesters with ethylenically unsaturated monomers, such as styrene, and to heat the resulting mixture, in the presence of a catalyst, such as a free radical generating substance, to effect copolymerization. This type of reaction has been discussed in numerous patents including that of Ellis, mentioned above, and in the scientific literature e.g., Industrial and Engineering Chemistry, Dec. 1939, page 1512.
It has been suggested to add to the above mixture of unsaturated polyester and copolymerizable monomer, an unpolymerizable or saturated linear polyester which mixture can be polymerized or cured to a substantially infusible state and thereby to produce products of improved tensile and flexural strengths, see for example U.S. Pat. No. 2,856,378 to Lundberg.
Such polyester compositions are often reinforced with fillers such as fiber glass either in the form of rovings or mats and the molded products prepared therefrom have found extensive application in industry for example in floor panels for, or portions of, automotive bodies, for appliance shells, boat hulls and the like.
However, in the formation of shaped articles with such reinforced plastic composition, difficulties arise because the fibers initially immersed within the uncured liquid resin tend to protrude from the surface causing the object, after molding and curing of the resin, to have irregularities on the surface. Moreover, when the compositions gel to form thermoset cured compositions shrinkage occurs, and the cured piece often develops very small cracks or "microcracks". Such shrinkage, microcracks, and surface irregularities are particularly undesirable where a molded article having a very smooth surface is required. Shrinkage results in the separation of minute areas of the surface of the cured product from the mold surface, with the result that the surface exhibits minute irregularities or unevenness and is not of the desired smoothness. The molded article fails to accurately reproduce the dimensions and surface characteristics of the mold and results in either rejection of the piece or costly finishing operations, e.g., multiple sanding operations and the like. Such irregularities in surface smoothness is readily determined by a profile examination of the surface and hence molded articles having a truly smooth surface have been termed "low profile" moldings. Moreover, the polymerizable polyester mixture is difficult to handle, being sticky or tacky. Thus, when processed into preformed items such as prepregs, sheet molding compounds or extruded rods, the preformed items are difficult to handle and store due to the tacky nature of the materials.
Various attempts have been made to solve the problems referred to above. Thus the problems caused by separation of the reinforcing fibers from the uncured resin have been reduced by chemical thickening of the uncured resin by the addition of magnesium oxide (USP 2,628,209) or calcium or magnesium hydroxide, (U.S. Pat. No. 2,568,331 to Frelette and U.S. Pat. No. 3,431,320 to Baum et al).
Although attempts have been made to decrease the shrinkage which such polymerizable compositions undergo upon curing, for example by incorporating in such compositions thermoplastic additives such as poly(methyl methacrylate), poly(vinyl chloride), saturated polyesters having hydroxyl terminal groups and the like, such attempts have not been completely successful. Thus they have either failed to significantly reduce cure-shrinkage or they have imparted to the cured product undesirable physical properties as low hardness, low impact resistance, poor paint adhesion and the like.
Moreover such compositions as indicated above are unsatisfactory for use in the recently developed sheet molding compound technique of molding reinforced plastic materials. In this development, the curable mixture is compressed or preformed between sheets of polyolefin, e.g., polyethylene or polypropylene film, and film stripped from the preformed sheets of the curable polyester compositions just prior to molding. In many instances, the polyolefin film cannot be removed cleanly from the polyester resin sheet due to the tacky nature of the latter, resulting from the inclusion of the hydroxy terminated saturated polyesters. This is highly undesirable since it complicates the removal of the polyolefin film. Inasmuch as it is conventional in this art, to prepare the sheet and store or ship it before molding, this tacky or sticky condition is obviously undesirable.
Accordingly, it is a principal object of this invention to provide polyester compositions suitable for the preparation of low profile molded articles which in the precured state are not tacky.
Other objects and advantages of this invention will be apparent from the following description of the present invention.